This invention relates to crystal resonators and, more particularly, to a high frequency crystal resonator with a resonating frequency on the order of 30 MHz or greater.
Crystal resonators i.e., monocrystalline resonators, are used in a variety of timing dependent applications, such as in computers. Computers are capable of executing multiple tasks simultaneously. Yet such execution typically involves sharing buses, memory, and other common structures. Computers are therefore synchronized to a high frequency clock signal to maintain data integrity. Crystal resonators are used in computers to generate the clock signals for maintaining synchronous operations.
The resonator is part of an oscillating circuit. The oscillator circuit generally comprises a piezoelectric crystal, a housing for protecting the crystal, and an amplifier-feedback loop combination capable of sustaining oscillation.
When a voltage is applied between certain faces of a piezoelectric crystal, a mechanical distortion is produced within the crystal. This phenomenon is known as the "piezoelectric effect". If the oscillator circuit is driven by an alternating current, the piezoelectric crystal is excited to a vibrating state at the frequency of the alternating current. When the oscillator circuit is energized, electrical noise will begin to excite the crystal at its natural resonant frequency. The crystal's output is then amplified and the amplified signal is fed back to the crystal. This causes the amplified signal to build up in strength at the resonating frequency of the crystal, until saturation of the circuit elements causes the overall loop gain in the circuit to fall to unity. This signal is fed to the output terminal of the oscillator.
Although a variety of piezoelectric materials may be used for a resonator, quartz crystal offers certain advantages. It has low internal mechanical loss when used as a vibrator. Another important feature of quartz is that its frequency of vibration is highly stable with changes in temperature and over long periods of time.
A resonator is formed from quartz by first cutting the quartz into slabs, grinding the slabs to a desired thickness by a lapping process, and then polishing the slab surfaces. The choice of cut is usually dictated by the range of operating frequencies and temperatures required for a particular application. Resonators with a particular oblique cut, such as AT, SC or BT, display negligible frequency variation with changes in temperature. These resonators are generally referred to as thickness shear resonators, and are useful for making high frequency oscillators on the order of 30 MHz or greater. The resonant frequency is approximately inversely proportional to the thickness of the wafer in the area of the vibration, so higher frequency devices require thinner wafers.
Single crystal quartz must be ground down to a very thin membrane to enable high oscillating frequencies. However, a thin membrane is a poor structure for attaching the resonator to the housing. It is therefore desirable to produce a resonator with both a vibrating membrane region and a thicker region, the latter region serving as a support structure for attachment purposes.
One such crystal resonating structure used in high frequency resonators is referred to as an "inverted mesa structure". Inverted mesa structure is a term of art referring to a crystal resonating structure having a thin central membrane completely surrounded by a thicker support structure. Electrodes are deposited on the membrane to allow the application of electrical energy to it to cause it to vibrate.
Inverted mesa structures have at least two disadvantages. First, the oscillating wave traveling outward from the electrode region of the membrane must diminish to a very low amplitude by the time it reaches the support structure. The membrane area must therefore be large relative to the electrode area to avoid undesirable damping of the resonance. Additional area is needed for the thicker supporting region, placing a physical constraint on the minimum size of the resonator.
Second, the fabrication process for inverted mesa structures is time-consuming and costly, since each crystal must be individually etched to the precise thickness necessary for high frequency applications.
The resonating circuit of a crystal oscillator typically comprises a pair of electrodes plated on opposing sides of the crystal. The anisotropy of AT crystal inverted mesa structures fabricated using liquid quartz etchants causes edges joining the membrane to the support structure to form irregular angles to the membrane's surface. When the angle formed is acute, the application of a continuous metallic layer from one level to the next becomes difficult.